This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-324178 filed on Oct. 24, 2000.
1. Field of the Invention
The present invention relates to a semiconductor dynamic quantity sensor capable of detecting a dynamic quantity in two axes.
2. Related Art
In recent years, demand for semiconductor two-axis sensors capable of detecting a dynamic quantity in two axes has grown. For example, in the air bag control system of a vehicle, acceleration should be detected in two axes corresponding to a head-on collision and a side collision.
If a sensor capable of detecting acceleration in one axis is employed for the system, at least two acceleration sensors are required. In this case, a larger space is required for disposing the two sensors, and further a control circuit is required to be installed for each of the two sensors. As a result, the problem that the entire system is large arises. It is advantageous to employ the above-described two-axis sensor as a solution of the problem.
JP-A-H9-318649 (U.S. Pat. No. 5,894,091) discloses a two-axis sensor. In the two-axis sensor, a movable electrode including a rectangular mass portion is formed in a semiconductor substrate. The mass portion is provided as a dynamic quantity sensitive part common to both the two axes. The movable electrode is connected to a stationary portion of the substrate by elastically deformable beam portions. Therefore it is movable in two axes which are mutually perpendicular on a plane parallel to the substrate.
Further four fixed electrodes are formed on the periphery of the movable electrodes. Thus, capacitors are formed between the movable electrode and the fixed electrodes for detecting displacement of the movable electrode in the two axes. However, the sensor is difficult to miniaturize, because the mass portion is rectangular and the fixed electrodes are arranged on the periphery of the rectangular mass portion.
Moreover, ideally, the mass portion should move only along the X-axis when acceleration is applied along the X-axis. However, actually, the vibration of the mass portion along the X-axis causes vibration along the Y-axis due to manufacturing variations of the beam portions. That is, the mass portion also vibrates along the Y-axis when acceleration is applied along the X-axis.
The vibration of the mass portion along the Y-axis is detected as high acceleration along the Y-axis, especially when the resonant frequency of the mass portion corresponding to the Y-axis is equal to that corresponding to the X-axis. In the case that one mass portion is provided as an acceleration sensitive part common to both the two axes, the equality between the resonant frequencies corresponding to the two axes is equivalent to the equality between the spring constants corresponding to the two-axes.
If mass portions are separately provided for the X-axis and the Y-axis, the resonant frequencies corresponding to the respective directions of the X-axis and the Y-axis differ from each other. However, the two mass portions occupy a relatively large area of the substrate.
It is therefore an object of the present invention to provide a semiconductor two-axis dynamic quantity sensor which can be readily miniaturized.
It is another object of the preset invention to provide a semiconductor two-axis dynamic quantity sensor in which the spring constants corresponding to two axes differ from each other.
A semiconductor dynamic quantity sensor according to the present invention includes a semiconductor substrate, a movable electrode, first fixed electrodes, and second fixed electrodes. The movable electrode includes a mass portion and electrode portions projecting from the center of the mass portion. The mass portion includes two rod portions which cross each other in an X-shaped configuration. Further, the movable electrode is connected to the substrate so as to be movable in response to a dynamic quantity applied thereto in a first direction and a second direction which are mutually perpendicular on a plane parallel to the substrate. The movable electrode is constructed so that a ratio of its resonant frequency corresponding to the second direction to its resonant frequency corresponding to the first direction is equal to or larger than 1.41.
The first fixed electrodes are supported by the substrate and disposed so that first capacitors for detecting displacement of the movable electrode in the first direction are formed between the first electrodes and the corresponding electrode portions. The second fixed electrodes are also supported by the substrate and disposed so that second capacitors for detecting displacement of the movable electrode in the second direction is formed between the second fixed electrode and the corresponding electrode portions.
Preferably, each of the electrode portions forms a comb-like shape projecting from the center of the mass portion. Further each of the first and second fixed electrodes also forms a comb-like shape, and is arranged so that its teeth are interdigitated with the teeth of the corresponding electrode portion. Each of the electrode portions and the first and second fixed electrodes includes a shortest tooth at a position furthest from the center of the mass portion and a longest tooth at a position proximate to the center of the mass portion.